Datasheet

G=
V V-
OUT1 OUT2
100mV 20mV-
V RTI(Referred-To-Input)=
OS
V
OUT1
G
- 100mV
0.40
0.36
0.32
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0
0
2
4 6 8
10 12 14 16 18
V (V)
OUT
V (mV)
SENSE
20
Actual
Ideal
INA270
INA271
SBOS381C –FEBRUARY 2007–REVISED MAY 2010 www.ti.com
Normal Case 2: V
SENSE
≥ 20mV, V
CM
< V
S
ACCURACY VARIATIONS AS A RESULT OF
V
SENSE
AND COMMON-MODE VOLTAGE
This region of operation has slightly less accuracy
than Normal Case 1 as a result of the common-mode
The accuracy of the INA270 and INA271 current
operating area in which the part functions, as seen in
shunt monitors is a function of two main variables:
the Output Error vs Common-Mode Voltage curve
V
SENSE
(V
IN+
– V
IN–
) and common-mode voltage, V
CM
,
(Figure 6). As noted, for this graph V
S
= 12V; for V
CM
relative to the supply voltage, V
S
. V
CM
is expressed
< 12V, the Output Error increases as V
CM
becomes
as (V
IN+
+ V
IN–
)/2; however, in practice, V
CM
is seen
less than 12V, with a typical maximum error of
as the voltage at V
IN+
because the voltage drop
0.005% at the most negative V
CM
= –16V.
across V
SENSE
is usually small.
This section addresses the accuracy of these specific
Low V
SENSE
Case 1:
operating regions:
V
SENSE
< 20mV, –16V ≤ V
CM
< 0; and
Low V
SENSE
Case 3:
Normal Case 1: V
SENSE
≥ 20mV, V
CM
≥ V
S
V
SENSE
< 20mV, V
S
< V
CM
≤ 80V
Normal Case 2: V
SENSE
≥ 20mV, V
CM
< V
S
Although the INA270 family of devices are not
Low V
SENSE
Case 1:
designed for accurate operation in either of these
V
SENSE
< 20mV, –16V ≤ V
CM
< 0
regions, some applications are exposed to these
Low V
SENSE
Case 2:
conditions. For example, when monitoring power
V
SENSE
< 20mV, 0V ≤ V
CM
≤ V
S
supplies that are switched on and off while V
S
is still
Low V
SENSE
Case 3:
applied to the INA270 or INA271, it is important to
know what the behavior of the devices will be in
V
SENSE
< 20mV, V
S
< V
CM
≤ 80V
these regions.
Normal Case 1: V
SENSE
≥ 20mV, V
CM
≥ V
S
As V
SENSE
approaches 0mV, in these V
CM
regions,
the device output accuracy degrades. A
This region of operation provides the highest
larger-than-normal offset can appear at the current
accuracy. Here, the input offset voltage is
shunt monitor output with a typical maximum value of
characterized and measured using a two-step
V
OUT
= 60mV for V
SENSE
= 0mV. As V
SENSE
method. First, the gain is determined by Equation 1.
approaches 20mV, V
OUT
returns to the expected
output value with accuracy as specified in the
(1)
Electrical Characteristics. Figure 17 shows this effect
using the INA271 (Gain = 20).
where:
V
OUT1
= Output Voltage with V
SENSE
= 100mV
V
OUT2
= Output Voltage with V
SENSE
= 20mV
Then the offset voltage is measured at V
SENSE
=
100mV and referred to the input (RTI) of the current
shunt monitor, as shown in Equation 2.
(2)
In the Typical Characteristics, the Output Error vs
Common-Mode Voltage curve (Figure 6) shows the
highest accuracy for the this region of operation. In
this plot, V
S
= 12V; for V
CM
≥ 12V, the output error is
at its minimum. This case is also used to create the
V
SENSE
≥ 20mV output specifications in the Electrical
Characteristics table.
Figure 17. Example for Low V
SENSE
Cases 1 and 3
(INA271, Gain = 20)
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